Replies

1. 
   DanH | Sep 03, 2007 08:15pm | #1

   If the arches are 2" thick, how do you get 6" of contact?

   Note that we have virtually no info here -- length or width of bridge, details on the shape of the arches, are the arches structural or cosmetic, etc.

   So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
1. 
   grpphoto | Sep 04, 2007 12:57am | #5

   Fine. The span of water is between 6' and 7', but the bridge footings are not directly opposite on the banks. That means that the bridge will have to be slanted across the water. I intend to use 8/4 oak for the beams. Each timber will start out being 12" wide, but I will bandsaw an arch 8" wide in the timber. I just finished the footings, I've been looking at the situation, and I think I will lay the beams out as a parallelogram, rather than a rectangle.I intend to use 4/4 or 3/4 oak for the planking. I will rip these boards about 3" wide and plane the ends to fit the curve of the arches. The bridge will be 32" wide.I will be happy to provide any other details. The basic question is - if I have 6" of beam resting on the stone on each side of the stream, is this secure enough?George Patterson

1. 
   MikeSmith | Sep 04, 2007 01:40am | #6

   george.... the 6" bearing should be fine

   but... you ought to reconsider the Oak arches..in that enviorment , the will be very susceptible to premature rot

   i'd think about using PT  LVL's for your arch.. you can use the Oak for the deck and the rails and pretty much have the whole thing look like it would if you used all oak

   the deck and rail components can be replaced if  they fail....but the arches should be designed for  a long  life

   but , hey, whadda i no ?Mike Smith Rhode Island : Design / Build / Repair / Restore

1. 
   grpphoto | Sep 04, 2007 03:55am | #8

   I want the oak for appearance, and white oak seems to hold up well. I would be willing to expend the money to use an epoxy finish, if that's what it takes, or maybe apply a thin layer or PT between the oak and the stone. Whadya think?George Patterson

1. 
   MikeSmith | Sep 04, 2007 05:29am | #10

   i think it's just like i said..  a structure like that can't be protected.. it's doomed to an early death

   i'd go with a  PT LVL arch.. and keep the oak for the visible componentsMike Smith Rhode Island : Design / Build / Repair / Restore

2. 
   peteshlagor | Sep 04, 2007 05:36am | #11

   I'd put a roof over it.

    

3. 
   MikeSmith | Sep 04, 2007 05:38am | #12

   yes... that's what saved the old onesMike Smith Rhode Island : Design / Build / Repair / Restore

4. 
   grpphoto | Sep 05, 2007 04:01am | #15

   I'll look into it. Perhaps I've been fooled by the English oak structures that seem to last forever.George Patterson

5. 
   brownbagg | Sep 05, 2007 04:46am | #16

   here what you need.Haga su trabajo de fricken

6. 
   FastEddie | Sep 05, 2007 04:51am | #17

   Mike sure has the experience to back up what he says, but I tend to agree that oak structures have stood the test of time.  I suppose part of that is the environment, including proper drainage, etc."Put your creed in your deed."   Emerson

   "When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it."  T. Roosevelt

7. 
   VaTom | Sep 05, 2007 06:22pm | #18

   Remember, no sapwood, all heart. 

   White oak's better, but I've got red oak treetops laying here in the woods from logging 35 yrs ago.  Sapwood's long gone but the heart's sound if there wasn't ground contact.  Hard on the chainsaw.

   I'd use native lumber, whatever's best.  In the Arizona mtns we had pondersosa pine last 30 yrs outside, similar application.  Considerably dryer there though.  Make sure you get good drainage.PAHS Designer/Builder- Bury it!

8. 
   sisyphus | Sep 06, 2007 01:13am | #19

   Sometimes when I have wood bearing on concrete (or stone )I put a piece of heavy galvanized mesh (hardware screen 1/2X1/2 or eqivalent) between the wood and concrete. I figure this prevents water from wicking up into the wood and if kept dirt free allows some air movement.  The places where moisture can remain for extended periods of time are the problem spots where rot occurs. I try to design to minimize these spots. Applying preservative before assembly gives one the oportunity to treat joints which are one of the major moisture retainers and hard to treat as effectively after. Assembling treated pieces is disgusting so I try to wear a barrier cream on my hands to minimize skin contact (or get my henchmen to do that part  LOL). You have a neat project.

9. 
   grpphoto | Sep 06, 2007 05:46am | #20

   Galvanized mesh, huh? Sounds good. I'm also thinking now about a 1" slab of oak laminated to the side of one of those PT beams that Mike was talking about. Might actually be cheaper. At least in materials costs.George Patterson

10. 
    sisyphus | Sep 07, 2007 01:46am | #21

    Using PT sounds like a good idea. Of course it  would probably mean that all metal elements in contact with it should be stainless steel (or copper?).

11. 
    grpphoto | Sep 07, 2007 05:32am | #22

    Been thinking about it, and I believe I can get the effect I want by laminating 1" oak to the outsides of the beams. Then I can use PT for the beam and keep the oak off the ground.Working with oak outside requires stainless hardware anyway. Regular steel will turn the stuff black as soon as it gets wet.George Patterson

2. 
   TomW | Sep 04, 2007 01:51am | #7

   What kind of oak are you using. It makes a big difference. White oak = good, red oak = bad for outdoor applications. The pore structure is completely different between the two.

1. 
   grpphoto | Sep 04, 2007 03:55am | #9

   White oak. I've heard the same as you.George Patterson

3. 
   DanH | Sep 04, 2007 06:13am | #13

   One remaining issue is how much motion you're going to see. The wood will swell and shrink seasonally, and the rocks may move as well.
   So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin

1. 
   grpphoto | Sep 05, 2007 03:58am | #14

   There will be about a 10" difference in elevation between the two ends of the bridge. I intend to put a concrete buttress between the lower end and the rising bank behind it. That will keep it from shifting in that direction. The slope of the bridge will keep it from shifting towards the other end.I've built several stone walls along this stream over the last few years; one of them about 7' tall. They've proven to be stable, so I'm gambling that this one will too.George Patterson

2. 
   PedroTheMule | Sep 03, 2007 08:35pm | #2

   More detail would be helpful; however based on what I "think" you are asking.....

   You would end up with 2" width by 6" of length resting on the stone.

   Depending upon the span, this should be more than sufficient.

   Poor comparison without additional details, but...a 2x10 can span 14' and only rest on an area about 1 1/2" x 2" and still carry the weight of whatever weighty furniture and people is placed upon it.

   So, "if" I follow your question.....go for it you'll be fine. 

    

   Pedro - NO not another straw on my back, what do you think I am, a camel?

3. 
   SamT | Sep 03, 2007 11:31pm | #3

   "The arches will be 2" thick"

   That is a typo, right? I mean

   "carved to fit the contours of the stone."

   wouldn't leave much meat to 2".

   SamT

1. 
   Sphere | Sep 04, 2007 12:26am | #4

   The way I cipher it, the arches=beams ( forget the curvature) and they will rest on irregular stone for a distance of 6".

   so total foot print is 12sq In. Per end.Spheramid Enterprises Architectural Woodworks

   "If you want something you've never had, do something you've never done"